What does harmony 1645 mean when arranging music?

1645 is the chord direction, 1 stands for 135, and C. 6 stands for 6 13, Am. 4 stands for 46 1, and F. 5 stands for 572 g. CDEFGAB means 1234567, 1 grade 135, grade 2 246, grade 3 357, grade 4 46 1, and grade 5 577. It's that simple! Music theory will make up if it doesn't pass! If you don't understand music theory, the work will not only be unpleasant to listen to, but also be laughed at by others.